It is now an established fact that changes in tau conformation occur during the course of Alzheimer's disease. The intricacies of these conformation changes, the timing of their appearance, and their effect on tau structure and its association into paired helical and/or straight filaments (PHF/SF) remain to be determined. During the first 4 years of this project, we have established that full-length tau begins its association into pathological structures by attaining the "Alz 50" conformation in which the extreme amino terminus of the molecule folds and associates with the microtubule binding domain. We have further established that the carboxy terminus of tau inhibits tau assembly in vitro. Immunohistochemical analysis of brain tissue sections using monoclonal antibodies that recognize precise carboxy terminal phosphorylation and truncation events indicate that these events occur early in AD. Moreover, we have modeled these events in our in vitro tau polymerization paradigm. We hypothesize that, although full-length tau is capable of forming tau filaments, truncation of the carboxy terminus of the tau molecule accelerates this process, driving the conversion from MCI to AD. This hypothesis will be tested by: 1) Identifying and characterizing additional truncated tau species using mass spectrometry; 2) Modeling the effect of specific truncations on tau's assembly into filaments in vitro; and 3) Producing monoclonal antibodies that recognize these truncations. We will then map the distribution of the truncations, immunohistochemically, employing both bright field and laser confocal microscopy on sections obtained from RADC and the ROS. The appearance of these markers on tau molecular alterations will be correlated with disease progression from no cognitive impairment (NCI), through mild cognitive impairment (MCI), to the final conversion to AD. Further comparisons will be made with immunohistochemical studies in RO8-01 in order to correlate the appearance of specific truncations and tau conformations with galanin hyperinnervation and the appearance of granulovacuolar degeneration bodies.